1. Field of the Invention
The present invention relates to an amplification circuit composed of a differential operational amplifier, and more specifically to an amplification circuit having a controllable slew rate which can be changed in accordance with a given necessary condition.
2. Description of Related Art
In the prior art, this type of amplification circuit comprises, as shown in FIG. 1, a differential operational amplifier 1 internally containing a current source circuit 10, switches SW71 to SW73, and capacitors C71 and C72, as basic amplification circuits. A slew rate of this amplification circuit is determined by a current value generated in the internal current source circuit 10 on the basis of a fixed output value of a constant current source circuit 70 connected to the internal current source circuit 10. In addition, the shown amplification circuit comprises a signal input terminal TIS7 for receiving an input signal, a reference voltage input terminal TIV7 for receiving a reference voltage, and a signal output terminal TOS7 for outputting an output signal.
As shown in the drawing, an inverted input and an output of the differential operational amplifier 1 are interconnected through a parallel circuit composed of the switch SW1 and the capacitor C71. The inverted input of the differential operational amplifier 1 is connected to one end of the capacitor C72 having the other end connected to the signal input terminal TIS7 through the switch SW72 in series to the capacitor C72. A non-inverted input of the differential operational amplifier 1 is connected to the reference voltage input terminal TIV7, and the output of the differential operational amplifier 1 is connected to the output terminal TOS7. The switch SW73 is connected between the non-inverted input or the differential operational amplifier 1 and the other end of die capacitor C72. Thus, the basic amplification circuit is constructed.
Furthermore, as shown in the drawing, the internal current source circuit 10 internally contained in the differential operational amplifier 1 is connected to a output of the constant current source circuit 70, so that the value of a current flowing through the internal current source circuit 10 remains constant. In this construction, a slew rate of the differential operational amplifier 1 changes in accordance with the value of the current flowing through the internal current source circuit 10. The larger the value of the current is, the larger the slew rate is. Therefore, in order to obtain a large slew rate, the constant current source circuit 70 outputting a large current value is used.
Now, an operation of the amplification circuit shown in FIG. 1 will be described.
When the switches SW71 and SW72 are in a closed condition and the switch SW73 is an open condition, the capacitor C71 is reset, and the capacitor C72 is charged with electric charges of the amount corresponding to an input voltage applied to the signal input terminal TIS7.
In a next period, the switches SW71 and SW72 are brought into an open condition and the switch SW73 is brought into a closed condition, so that the electric charge stored in the capacitor C72 is re-distributed between the capacitor C71 and the capacitor C72, and on the signal output terminal TOS7 there appears an output voltage in accordance with the value of the voltage applied to the signal input terminal TIS7 and the ratio in capacitance between the capacitor C71 and the capacitor C72.
In the prior art amplification circuit mentioned above, since the internal current source circuit internally contained in the differential operational amplifier is connected to the output of the constant current source circuit, and therefore, since the slew rate of the differential operational amplifier changes in accordance with the value of the current flowing through the internal current source circuit, a constant current source circuit capable of giving a large current is always required in order to temporarily obtain a large slew rate. Namely, in order to maintain the large slew rate, a consumed electric power of the circuit inevitably increases.